The aforementioned carbon nanotubes have a structure consisting of a cylindrically rolled network of six-membered rings of carbon element. They can be roughly classified into single-walled carbon nanotubes and multi-walled carbon nanotubes according to the number of graphene sheets (i.e. the number of walls) forming a single tube. Multi-walled carbon nanotubes with two or three walls may also be called double-walled or triple-walled carbon nanotubes, respectively.
Due to their simple structures, single-walled carbon nanotubes have enjoyed more rapid progress in theoretical analyses than multi-walled types. It has been theoretically suggested that they exceed any existing materials in terms of thermal conductivity, elastic coefficient, tensile strength and allowable current density, and have ballistic electron conductivity as well as semiconductor properties. Currently, many of those properties are being experimentally demonstrated and developed into application studies.
An ultrafine carbon fiber that is not tubular but a simple fiber is called a carbon nanofiber and should be distinguished from carbon nanotubes (especially, single-walled carbon nanotubes). This is because the aforementioned characteristics that are expected from carbon nanotubes result from their tubular structure and cannot be achieved by carbon nanofibers that simply consist of ultrafine carbon fibers.
There are several known methods for the synthesis of carbon nanotubes, such as arc discharge, laser evaporation and chemical vapor deposition (CVD). Among these methods, arc discharge is expected to be suitable for the synthesis of carbon nanotubes since this technique is capable of synthesizing a larger amount of carbon nanotubes than laser evaporation and is also superior to CVD in terms of the crystallinity of the resulting nanotubes.
A process of synthesizing carbon nanotubes by arc discharge includes the steps of filling a vacuum chamber with an inert gas (e.g. helium or argon) or hydrogen-containing gas (e.g. hydrogen, or hydrogen sulfide) to a pressure of approximately 100 to 500 Torr and then generating an arc discharge between the carbon electrodes facing each other within the vacuum chamber to evaporate the carbon electrode on the positive side, which contains carbon and a catalyst metal. It is known that a substance deposits on the cathode during this synthesizing process. This substance is called the cathode deposit.
Positive carbon electrodes used in the arc discharge method can be roughly classified into pure carbon electrodes with no metal content and metal-carbon composite electrodes containing a catalyst metal. Using a pure carbon electrode as the anode tends to result in the formation of multi-walled carbon nanotubes in the substance deposited on the cathode, whereas using a metal-carbon composite electrode as the anode is likely to cause a deposition of single-walled carbon nanotubes onto the inner wall surface of the chamber. For the catalyst metal used in the metal-carbon composite electrode, the iron-group elements (e.g. Fe, Ni or Co) are commonly used. It is said that adding 0.3 to 5 mol % of an iron-group element enables the element to act as the catalyst to produce carbon nanotubes (refer to Non-Patent Document 1 to be mentioned later).
It is also known that the amount of the resulting single-walled carbon nanotubes can be increased by using a binary system of Ni and Y as the catalyst metals rather than using only an iron-group element (refer to Non-Patent Document 2 to be mentioned later).
In addition, it is known that, if a metal-carbon composite electrode containing iron is used as the anode and a hydrogen gas as the atmospheric gas in the arc discharge, the resulting carbon material will barely contain amorphous carbon (i.e. the percentage of carbon nanotubes will be high) because hydrogen removes amorphous carbon (refer to Non-Patent Document 3 to be mentioned later).
As just explained, the production of single-walled carbon nanotubes by arc discharge requires a catalyst metal. However, this catalyst metal must be treated as an impurity when the carbon nanotubes are to be used after purification. Accordingly, it is necessary to adequately purify the carbon nanotubes by removing the impurities so as to isolate only the carbon nanotubes and improve their basic properties for various applications. As such purification techniques, the following methods have been known:
(1) Wet purification, in which the catalyst metal is dissolved in an aqueous acid solution, such as an aqueous solution of hydrochloric acid or sulfuric acid (refer to Patent Document 1 to be mentioned later).
(2) Heat treatment, in which the temperature is raised to a level higher than the boiling point of the catalyst metal to evaporate the catalyst metal away (refer to Non-Patent Document 4 to be mentioned later).    Patent Document 1: JP-A 8-198611    Non-Patent Document 1: Yahachi Saitou and Shunji Bandou, Kaabon Nanochuubu No Kiso (Fundamentals of Carbon Nanotubes), Corona Publishing Co., Ltd., 1998    Non-Patent Document 2: C. Journet et al., Nature, 388, 1997, 756-758    Non-Patent Document 3: X. Zhao, Chem. Phys. Lett, 373, 2003, 266-271    Non-Patent Document 4: Carbon, 41, 2003, 1273-1280